Multichannel near-field nanoscopy of circular and linear dichroism at the solid-state

We investigate the optical response of individual porphyrin (TPPS3) nanoaggregates anchored onto a glass substrate by using a specific configuration of Polarization-Modulation Scanning Near-Field Optical Microscopy (PM-SNOM). Sub-diffraction spatial resolution and sensitivity to the chiroptical properties of the material is reported. By demodulating the transmitted signal at the first and the second harmonics, the response of the nanoggregates to circular and linear polarization is simultaneously acquired in the same scan. In order to evaluate the relevant dichroic coefficients, we analyze a sample comprising several tens of individual nanoaggregates by using a model based on the Mueller matrix formalism. Circular dichroism in the nanoaggregates is demonstrated to stem from their molecular structure, whereas linear dichroism occurs as a consequence of the strongly anisotropic shape of the deposited nanoaggregates.